1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to video encoding and decoding and, more particularly, to encoding and decoding a video signal according to a directional intra-residual prediction.
2. Description of the Prior Art
Since multimedia data that includes text, moving pictures (hereinafter referred to as “video”) and audio is typically large, mass storage media and wide bandwidths are required for storing and transmitting the data. Accordingly, compression coding techniques are required to transmit the multimedia data. Among multimedia compression methods, video compression methods can be classified into lossy/lossless compression, intraframe/interframe compression, and symmetric/asymmetric compression, depending on whether source data is lost, whether compression is independently performed for respective frames, and whether the same time is required for compression and reconstruction, respectively. In the case where frames have diverse resolutions, the corresponding compression is called scalable compression.
The purpose of conventional video coding is to transmit information that is optimized to a given transmission rate. However, in a network video application such as an Internet streaming video, the performance of the network is not constant, but varies according to circumstances, and thus flexible coding is required in addition to coding optimized to the specified transmission rate.
Scalability refers to the ability of a decoder to selectively decode a base layer and an enhancement layer according to processing conditions and network conditions. In particular, fine granularity scalability (FGS) methods encode the base layer and the enhancement layer, and the enhancement layer may not be transmitted or decoded depending on the network transmission efficiency or the state of the decoder side. Accordingly, data can be properly transmitted according to the network transmission rate.
FIG. 1 illustrates an example of a scalable video codec using a multilayer structure. In this video codec, the base layer is in the Quarter Common Intermediate Format (QCIF) at 15 Hz (frame rate), the first enhancement layer is in the Common Intermediate Format (CIF) at 30 Hz, and the second enhancement layer is in the SD (Standard Definition) format at 60 Hz. If CIF 0.5 Mbps stream is required, the bit stream is truncated to obtain a bit rate of 0.5 Mbps based on a first enhancement layer having a CIF, a frame rate of 30 Hz and a bit rate of 0.7 Mbps. In this method, spatial and temporal SNR scalability can be obtained.
As shown in FIG. 1, frames (e.g., 10, 20 and 30) of respective layers, which have the same temporal position, have images similar to one another. Accordingly, a method of predicting the texture of the current layer and encoding the difference between the predicted value and the actual texture value of the current layer has been proposed. In the Scalable Video Mode 3.0 of ISO/IEC 21000-13 Scalable Video Coding (hereinafter referred to as “SVM 3.0”), such a method is called intra-BL prediction.
According to SVM 3.0, in addition to an inter-prediction and a directional intra-prediction used for prediction of blocks or macroblocks that constitute the current frame in the existing H.264, a method of predicting the current block by using the correlation between the current block and a corresponding lower-layer block has been adopted. This prediction method is called an “intra-BL prediction”, and a mode for performing an encoding using such a prediction method is called an “intra-BL mode”.
FIG. 2 is a view schematically explaining the above-described three prediction methods. First ({circle around (1)}) intra-prediction with respect to a certain macroblock 14 of the current frame 11 is performed, second ({circle around (2)}) inter-prediction using a frame 12 that is at a temporal position different from that of the current frame 11 is performed, and third ({circle around (3)}) intra-BL prediction is performed using texture data for an area 16 of a base-layer frame 13 that corresponds to the macroblock 14.
In the case of encoding residual data by obtaining the difference between the result of the prediction and a video to be encoded according to the result of the prediction in the temporal inter-prediction, the compression efficiency is increased. In addition, the compression efficiency can be heightened by reducing the amount of data to be encoded by obtaining the difference between the residual data. Consequently, a method and an apparatus for compressing the residual data in the directional intra-prediction are required.